asp.net error form cannot be nested within element form? -

i have content page in asp.net application uses form tag. there's 1 on page i'm confused why give me error: validation (html5): element 'form' must not nested within element 'form'

heres code:

<%@ page language="c#" autoeventwireup="true" masterpagefile="~/site.master" codebehind="default.aspx.cs" inherits="webapplication6._default" %> <asp:content id="content1" runat="server" contentplaceholderid="maincontent">    <div>       <form id="form1">          <asp:gridview id="gridview1" runat="server" allowsorting="true" autogeneratecolumns="false" datakeynames="id" datasourceid="sqldatasource1" allowpaging="true" onselectedindexchanged="gridview1_selectedindexchanged">             <columns>                <asp:boundfield datafield="id" headertext="id" insertvisible="false" readonly="true" sortexpression="id" />                <asp:boundfield datafield="title" headertext="title" sortexpression="title" />                <asp:boundfield datafield="director" headertext="director" sortexpression="director" />                <asp:boundfield datafield="datereleased" headertext="datereleased" sortexpression="datereleased" />                <asp:templatefield headertext="selection">                   <itemtemplate>                      <asp:checkbox id="selections" runat="server" oncheckedchanged="checkbox1_checkedchanged1" />                   </itemtemplate>                </asp:templatefield>             </columns>          </asp:gridview>          <asp:sqldatasource id="sqldatasource1" runat="server" connectionstring="<%$ connectionstrings:connectionstring %>" selectcommand="select * [movies]"></asp:sqldatasource>          <asp:button id="button1" runat="server" text="select movies" onclick="button1_click" causesvalidation="false" />          <asp:textbox id="textbox1"  text="first name" runat="server"></asp:textbox>       </form>    </div> </asp:content> 

i have form in masterpage wasnt giving me problems yesterday..

any ideas?

if <asp:contentplaceholder id="maincontent" > control is, itself, inside of form element, shouldn't place form inside of asp:content control should not have nested forms.

from html5 working draft:

4.10.3 form element
content model:
flow content, no form element descendants.

update

see question a page can have 1 server-side form tag:

master pages should not contain form tags in general because meant used base layout of content page.

try restructure project using these guidelines:

• only add form elements aspx pages
• add main content masterpage pages
• add content needs nested within form usercontrol placed within page.